1. Field of the Invention
The present invention relates to a transmission control apparatus and a transmission control method for transmitting information by radio communication toward a mobile station located in a predetermined sector.
2. Related Background Art
It is possible to increase or decrease an antenna gain in a specific direction by arranging a plurality of dipole antennas on an array and adaptively controlling weighting factors (weights) of the respective antennas by signal processing. This process of adjusting the antenna gain in the specific direction to form a directional beam is a very useful technique in the radio transmission systems. This is because the technique enables transmission with reduced transmission power by increasing the antenna gain in the desired direction, and reduction of interference with signals transmitted in the other directions by decreasing antenna gains in non-desired directions, in transmission of signals.
The technique is also under active research and development for implementation in W-CDMA (Wideband Code Division Multiple Access) being one of radio access systems of the third-generation mobile communication systems, by making use of the principle of the directional beam transmission as discussed; for example, reference is made to Document “A. Harada, S. Tanaka, M. Sawahashi, and F. Adachi, “Performance of Adaptive Antenna Array Diversity Transmitter for W-CDMA Forward Link,” Proc. PIMRC99, pp. 1134-1138, Osaka, Japan, September 1999.” This study presents a method of signal transmission from abase station to mobile stations in the mobile communication system, in which antenna directivities for respective mobile stations adaptively generated in the base station are multiplied with transmit signals to the respective mobile stations to provide the directivity toward each desired mobile station and reduce interference with signals in directions toward the other mobile stations, and investigation is done on a technique of increasing subscriber capacity.
In the investigation of transmission with the antenna directivity as described above, however, it is the present status that many studies are directed toward improvements in the subscriber capacity in comparison with the omnisector transmission to transmit signals to the area (sector) of the base station without execution of the directional beam transmission, and there are no studies made on selective use of the transmission methods of omnisector transmission and directional beam transmission and on allocation to the respective transmission methods according to types of transmitted signals.
Particularly, in the future radio transmission systems, it is believed that the mainstream will turn from the circuit switching type signal transmission in which signals to be transmitted like voice are continuously present, to the data transmission in which signals appear burstwise like data, and that packet transmission suitable for the burst transmission will be performed. In the packet transmission signals appear neither constantly nor continuously and the required quality and amount of information vary according to types of data.
It is thus very important even in application of the directional beam transmission to select an appropriate transmission method to transmit signals, according to the type of information allowing for the required quality of burstwise appearing signals and the information amount.
The present invention has been accomplished in order to solve the above problem and an object of the invention is to provide a transmission control apparatus and a transmission control method capable of transmitting signals by an appropriate transmission method according to a type of information even in the case of the directional beam transmission being applied.